Introduction

it is 3x the price and almost twice as heavy as the full frame Canon EF 200mm f/2.8L lenses, but then the Panasonic is a vastly superior lens and adds:

weathersealing

much improved sharpness in order to match the MFT's higher pixel density and the effective 400mm field of view

much faster and more accurate AF - more sophisticated AF motor system with silent much faster AF in Live View / Video, and able to do face AF

optical image stabiliser and Dual IS when used with compatible Panasonic cameras

presumably less vignetting as the filter size is 77mm not 72mm

9 rounded diaphragm blades instead of 8 non-rounded blades

nano lens coating

focus limiter switch

closer focusing to 1.15m instead of 1.5m

in essence, it functions in a similar role as the Canon EF 400mm f/5.6L USM lens which gives the same DOF, and field of view on a full frame dSLR but you would need to use the dSLR at 2 stops higher iSO to make up for the loss of aperture, and compared to this lens, the Panasonic is:

2.5x the price but you are getting the 1.4x TC thrown in

same weight but better weathersealing

more effective Dual IS (6.5EV on a compatible Panasonic body)

much shorter at 174mm compared with 257mm

less conspicuous black instead of white

nano coating

much improved sharpness in order to match the MFT's higher pixel density

faster, more accurate AF - more sophisticated AF motor system with silent much faster AF in Live View / Video, and able to do face AF

9 rounded diaphragm blades instead of 8 non-rounded blades

much closer focusing to 1.15m instead of 3.5m

issue with AF on Olympus cameras - ensure you update the camera firmware however, with latest firmware of Olympus OM-D E-M1 Mark II, this is fixed and the lens works in ProCapture mode, and the lens memory button works. The OIS gives sharp images down to 1/50th sec when using the 1.4x TC - but the Olympus 300mm f/4 can be used to 1/13th sec thanks to its Dual IS capability. The images with 1.4x TC are almost as sharp as the Olympus 300mm f/4.

sensationally sharp in the centre wide open - almost diffraction limited resolution (sharpest in the centre at f/2.8 compared to other apertures), edges are very sharp wide open (sharper than most other lenses in the centre wide open) but sharpest at f/4

AF is fast - 0.2secs infinity to close without OIS and 0.5sec with OIS

only 0.5EV vignetting wide open and very low distortion levels

best stopped down to f/8 to attain higher sharpness levels

gives slightly more lateral CA than the Pan. 100-400mm lens at 400mm

compared to full frame equivalents

a 400mm f/5.6 full frame lens will provide the same depth of field (DOF), field of view and similar low light performance (as FF sensor 2 stops higher ISO performance which counterbalances the 2 stops of aperture difference)

alternatively, a similarly sharp 200mm f/2.8 lens on a 80mp full frame camera cropped to a 2x crop will give the same image quality, DOF and FOV as the Panny on a 20mp MFT camera

Canon EF 400mm f/5.6L lens

the old Canon lens is NO MATCH for the Panasonic!

the Panasonic lens:

is sharper optically with less aberrations

is marginally lighter (Canon is 1.35kg)

is much shorter (Canon is 257mm long)

focuses much closer (Canon only focuses to 3.5m compared with 1.15m)

focus limiter more useful starting at 3.5m rather than 8.5m

is much better image stabilized - OIS and Dual IS (Canon has no OIS and no Canon dSLR nor the Canon R has IBIS)

has 9 rounded diaphragm blades (Canon has 8 straight blades)

has much better AF mechanism optimised for mirrorless systems

Sony FE mount

the only 400mm prime in Sony FE mount is the super expensive and heavy 400mm f/2.8

Sony FE 100-400mm f/4.5-5.6 GM OSS lens

the Sony has even closer focus to 0.98m and the focus limiter starts at 3m

the Sony is similar size and weight as the Canon 400mm f/5.6L (see above) BUT it EXTENDS on zooming to the 400mm

the Sony's edges are soft at 400mm and need to shoot at f/11 while CA is worst at 400mm and there is 1.5% distortion at 400mm, but it is sharper than any of its full frame 100-400mm zoom peers

Canon R mount

must use the Canon EF lenses as no native lenses yet

Nikon Z mount

must use the Nikon F lenses as no native lenses yet

compared to APS-C equivalents

equates to a 250mm f/4 lens to give the same depth of field (DOF), field of view and low light high ISO performance, but no such lens exists for APS-C or FF formats

a 200mm f/2.8 with a 1.4x teleconverter would give 280mm f/4 but it would be much less sharp than the Panny

Canon EF 300mm f/4L IS lens

the 300mm gives a little more telephoto reach (480mm FF lens eq), weighs 1.3kg, 221mm long, close focus 1.5m, 8 straight blades, the OIS is only 2EV and is noisy, and focus limiter is from 3m

this lens could be used on a full frame camera in 1.3x crop mode to give similar field of view as the Panny but with slightly wider effective aperture and slightly shallower DOF, but would it be as sharp?